docs: def _write_txt(self, stream):
          '''
          Save a PLY element to an ASCII-format PLY file.  The element may
          contain list properties.
  
          '''
          for rec in self.data:
              fields = []
              for prop in self.properties:
                  fields.extend(prop._to_fields(rec[prop.name]))
  
              _np.savetxt(stream, [fields], '%.18g', newline='\n')